With respect to photographic images composed of silver, the ratio of optical density of images to the amount of silver composing the images per unit area is generally called covering power, which is a measure for valuation of optical efficiency of silver composing the images. The covering power of the silver halide photographic light-sensitive layer increases generally with reduction of the grain size of silver halide grains and decreases as the grain size increases. On the other hand, since the sensitivity of the silver halide emulsion layer generally becomes high with the increase of the grain size of silver halide grains, silver halide emulsions having a large grain size are used for producing photographic light-sensitive materials having a high sensitivity. Accordingly, the photographic light-sensitive materials having a high sensitivity require a large amount of silver per unit area in order to obtain a certain image density. In other words, it is needed for the material to contain a larger amount of silver salts per unit area of the photographic light-sensitive material in order to obtain both the high sensitivity and the required maximum image density. This is the actual state that the prior photographic light-sensitive materials having a high sensitivity are encountered.
One attempt at improving the covering power while maintaining high sensitivity involves adding various polymers to a high-speed coarse silver halide emulsion. Such techniques have been described in British Pat. Nos. 1,048,057 and 1,039,471 and U.S. Pat. Nos. 3,043,697 and 3,446,618. They are all insufficient, because though the covering power is improved more or less, the strength of the coating film deteriorates. Particularly, in an automatic developing machine which is now conventionally used, when a photographic material having a coating film of poor strength is used, part of the gelatin in the film dissolves in a developing solution or a fixing solution and adheres to conveying rolls in the automatic developing machine. Accordingly, pollution of photographic images is caused by transferring to the light-sensitive material.
On the other hand, U.S. Pat. Nos. 2,996,382 and 3,178,282 have disclosed that photographic images having a high contrast and a high covering power are obtained at high sensitivity using silver halide photographic light-sensitive materials wherein surface latent image type coarse silver halide grains and fine silver halide grains having fog nuclei in the inside thereof are contained in the same layer or in adjacent layers.
However, this method is not desirable because development requires a long period of time in conventional low temperature treatment in order to obtain a sufficiently high density and a high contrast at high sensitivity, and the desired effect cannot be obtained with conventional high temperature rapid treatment.
Attempts at eliminating these disadvantages involve introducing various additives such as rhodan, imidazoles, thioethers, etc. into light-sensitive materials or processing solutions. (For example, U.S. Pat. No. 2,996,382 and Japanese Patent Application (OPI) Nos. 78535/82 and 89749/82. The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) However, it is obvious that any means for improving the above described disadvantages without adding the additives is more preferred.